The production of long-lived, high-affinity antibodies is critical for resistance to infection. This process requires the formation of germinal centers. Dysregulation of germinal center formation results in immunodeficiencies and autoimmunity. Elucidation of the regulatory mechanisms that govern germinal center formation is therefore essential to understanding clinical immunodeficiencies and autoimmune disorders. Germinal center formation requires antigen-stimulated B cells to migrate within the B cell follicle where they interact first with cognate T cells and then with follicular dendritic cells. The complex migration of activated B cells during germinal center formation is guided by chemotactic and adhesive cues, but the molecular regulation of B cell motility remains poorly understood. This application proposes to define the role of the actin-bundling protein L-plastin in integrating the chemotactic and adhesive cues essential for normal B cell motility, and by extension, germinal center formation. I have shown that the actin-bundling protein L-plastin is essential in B cell motility and for development of specialized marginal zone B cells. Deficiency of L-plastin disrupts maintenance of protein levels and activation of the integrin-associated kinase Pyk2. Germinal center formation and the production of class-switched antibodies following antigenic stimulation also require L- plastin. In Aim 1, we will define the structural domains of L-plastin that are essential for function in B cell motility and marginal zone B cell maturation using an established lentiviral expression system. Candidate domains to be tested first are the N-terminal serine phosphorylation site, the calcium-binding domain, and the actin-bundling domains. This Aim will determine if the actin-bundling function of L-plastin is essential to its function in B cel motility, or if L-plastin serves as an adaptor protein in chemokine/integrin signaling independently of actin-bundling, and will define mechanisms by which upstream mediators may modulate the function of L-plastin. In Aim 2, we will analyze chemokine and integrin signaling upstream and downstream of Pyk2 activation to delineate the role of L-plastin in chemotactic and adhesive signaling cascades. We will also determine the mechanism by which Pyk2 protein levels are maintained in B cells. In Aim 3, newly validated mice expressing a conditional L-plastin allele will be used to generate mice with a B cell-specific deletion of L- plastin. Analysi of the development of humoral immunity in mice lacking L-plastin only in B cells will define the steps of germinal center B cell formation most dependent on chemotactic cues. I have the expertise required to conduct these experiments and all the reagents necessary to complete the proposal in hand. These Aims will establish L-plastin as a key regulator of humoral immunity and provide insight into the integration of chemotactic and adhesive signaling cascades essential for antibody generation. Definition of the molecular regulation of B cell motility is essential to understanding the pathogenesis of immunodeficiency and autoimmunity, as well as to designing and improving vaccines and immunomodulatory drugs.